The invention pertains to the spectral analysis of light.
The currently used procedures for the spectral analysis of light can be divided into three groups. The first and oldest approach is based on the angular spectral separation of light. The light, upon interaction with a suitable optical element (optical prism, optical grating), changes the direction of propagation. This direction depends on the wavelength. Such spectrally separated components can be independently analyzed. The second, frequently used procedure is based on the phenomenon of light interference when a spectrum of the analyzed radiation can be calculated from the interferrogram. The third, recently published procedure uses the phenomenon of the dispersion of the optical rotation (CZ Patent 284 282). When the linearly polarized light passes through a proper element (rotator), its polarization plane rotates as a function of wavelength. This can be used for measurement of its spectrum.
The subject of the invention is a new method for measurement of spectral characteristics of light originating at a distant point light source, object or at two dimensional scene. The method is based on the phenomenon of dispersion of the optical rotation. The invention is a modification of the method patented in CZ 284 282. The principle of the method is illustrated in FIG. 1. The analyzed beam of radiation 2 emerging from the aperture 1 is converted by passing through optical system 3 and polarizer 5 to a linearly polarized parallel beam of radiation 6. This collimated polarized radiation is subjected to the optical rotation in the environment with a dispersion of the optical rotation (rotator) 7. The extent of this rotation is selected according to the spectral interval in which the spectral analysis is being done. It means that the optical rotation of the rotator can be with an advantage adjusted differently for broad-band and for narrow-band radiation, as well as for radiation from different spectral regions, i.g. for uv, vis or ir radiation. The rotator outputs radiation 8 having polarization planes of the individual spectral components rotated. The extent of the rotation depends on the wavelength of the spectral component. Such parallel beam passes through the second polarizer 9 (analyzer). The polarization plane 17 of the analyzer forms an angle xcfx86=xcfx861 with the polarization plane of the analyzed radiation 16. The beam 10 is focused by the optical system 11 on the detector 13. Then, the intensity S of the output beam 10 is measured by a single or a multichannel detector. The procedure is repeated with fixed parameters of the rotator for a set of angles xcfx86 from the interval (xcfx861,xcfx862). Such procedure yields functional dependence of the output intensity S(xcfx86) on the angle xcfx86. Spectral characteristics of the analyzed radiation are then extracted from this dependence by a mathematical analysis. The spectral characteristics obtained can be either a full spectrum, or some characteristic spectral parameter only, for example a location of the spectral maximum, a spectral half-width, etc.
When spectral characteristics of a point light source are measured, the collimated polarized radiation 6 passes through the optically active environment 7 that exhibits the same dispersion of the optical rotation in the entire crossection of the analyzed beam. The output light beam 8 is then directed on a single-channel detector which measures the light intensity S(xcfx86).
When spectral characteristics of a planar light source 18, FIG. 2, or of a distant scene are measured, the collimated polarized radiation 6 passes through the optically active environment 7 that exhibits the same dispersion of the optical rotation in the entire crossection of the beam and the output intensity 8 of the light is focused by the optical system 11 on a multi-channel detector 20. Each pixel of the detector measures intensity of a corresponding point in the field of view. This process is known as imaging.
The device according to the presented method consists of a collimating optical system 3 and a detector 13, FIG. 1. Importantly, the first polarizer 5, rotator 7, and the second polarizer 9 are placed between the optical system 3 and the detector 13. The rotator must exhibit a non-zero dispersion of the optical rotation and its parameters affecting the amount of the optical rotation experienced by the light do not change during the measurement. When collimated or polarized radiation is analyzed, the device does not require the collimating optical system 3 or the first polarizer 5, respectively.
An advantage of the method for the spectral analysis of light and an advantage of the device using the new method is that, in comparison with the previously published method, the parameters of the rotator do not change during the measurement. Only the second polarizer 9 rotates instead.
For the identification of spectral characteristics of the input radiation according to the new method, the output intensity is measured for a set of different angles xcfx86 of the analyzing polarizer. For collimated, polarized, polychromatic beam of light 6 with wavelengths from the spectral interval (xcex1,xcex2), the intensity S(xcfx86), after passing through the rotator 7 and through the analyzer 9, is measured as a function of the angle xcfx86 from the interval (xcfx861,xcfx862):                               S          ⁢                      (            ϕ            )                          =                              ∫                          λ              1                                      λ              2                                ⁢                                    I              ⁢                              (                λ                )                                      ⁢                                          cos                2                            ⁡                              [                                                      θ                    ⁢                                          (                      λ                      )                                                        -                  ϕ                                ]                                      ⁢                          ⅆ              λ                                                          (        1        )            
where I(xcex) is a spectrum of the input radiation, xcfx86 is an angle 16 between the polarization plane of the analyzer and a polarization plane of the linearly polarized input light, and xcex8(xcex) is a rotation angle 15 of the polarization for the radiation with wavelength xcex after a passage through the rotator. The function xcex8(xcex) is known for a given rotator and depends on geometric and material properties of the rotator. Since the function S(xcfx86) has a period xcfx80, measurements for angles xcfx862-xcfx861 greater than xcfx80 do not provide any new information and do not need to be done. Characteristics of the spectrum I(xcex) are determined from the dependence of S(xcfx86) on xcfx86 described by Eq. (1). Depending on the complexity of the function I(xcex), it can be either the full spectrum, or some of its spectral parameters, e.g. wavelength of the spectral maximum, spectral half-width, etc.
In the device that works according to the above presented principle, the linearly polarized parallel beam of radiation 6 first passes through the optical rotator 7 where the polarization planes of the individual spectral components of radiation rotate in dependence on their wavelengths, further passes through the analyzer 9, and then the radiation 12 impinges on the detector which measures intensity S(xcfx86) as a function of the rotation angle xcfx86 of the analyzing polarizer 9. Characteristics of the spectrum I(xcex) are then determined from the formula for S(xcfx86), Esq. (1).